Automatic telephone system



' Dec. 8, 1925; 1,564,487

T. G.-MARTIN AUTOMATIC TELEFHONE SYSTEM F'iled llay 10. 1923 2 S eets-Sheet 1 Dec. 8, 1925. 1,564,487

. T. G. MARTIN AUTOMATIC TELEPHONE SYSTEM Filed May 10, 1923 2 2 Sheets-Sheet &

-- u en 1:11:

Patented Dec. 8 1925.

UNITED STATES PATENT .QF'FICE'.

TALBOT G. MARTIN, OF CHICAGO, ILLINOIS, ASSIGNOB TO AUTOMATIC ELECTRIC COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS.

AUTOMATIC TELEPHONE SYSTEM.

' Application filed May 10, 1923. Serial No. 637,931.

To all" whom it may concern: Be it known that I,'TALB0T G. Manrm, a citizen of the United States of America, and

. a resident of Chicago, Cook County, and

State of Illinois, have invented certain new and usefpl Improvements in Automatic Telephone Systems, of which the following is a specification.

The present invention relates to automatic telephone systems in general but is con-.

cerned more particularly with telephone systems in which the finder sw1tches are employed; and the principal object 1s the production of a new and improved finder system which is reliable in operation and which uses finder switches which are simple in construction.

More specificall an object of the invention is the suitab e application of a simple distributer switch to a starting circuit which is looped through a plurality of finder switches. This distributer switch steps ahead after each call and is employed to even up the traffic on the various finder switches and also to enable'a callin subscriber to get service eventually even though the starting loop does happen to be open at one or more points or in case one or more finders are out of order.

Another object of the invention is the production of a new and improved finder switch having multiple wipers for increasing its capacity.

Another object is the production of suitable circuit arrangements whereby a discriminating tone source is associated with a subscribers line at the exchange only when the line is in use.

The other objects are concerned mainly with various circuit details and refinements and will be explained fully hereinafter.

Referring now to the drawings comprising Figs. 1 and 2, they show by means of the usual circuit diagrams a sufficient amount of apparatus in a system embodying the rinciples of 'the invention to enable the invention to be thoroughly understood and,

steppin magnet 174, as will be explained also the finder F which has access to HEISSUED the lines of substations A and A, and which is assumed to be associated with a first selector switch through the medium of the three conductors extending- 'to the righthand edge of the drawing.

Figure 2 shows the common start wire distributer D and also the finder F whose banks are multipled with the. banks of the finder F.

The finders such as F and F are automatic switches of the well-known vertical and rotary type in which the bank contacts are arranged in horizontal rows or levels. Each of these switches has two separate sets of bank contacts and to separate sets of cooperating wipers, in 'order to double the normal capacity of 100 lines. Each finder also has a vertical test wiper and bank, as shown by- Smith and Campbell in Automatic Telephony, second edition, Figs. 65 and 66, pages 68 and 69.

The line circuit LC comprises a two-magnet two-step combination line and cut-ofi' relay whose magnets are indicated by the reference characters 16 and 17. is of the same type as the one disclosed in the co-pending application of Erickson, filed June 17, 1921, Serial No. 478,377, i. e., it has one armature which is operated in two distinct steps, each step controlling a pluralit of armature-springs hereinafter referred to as such, or merely as armatures according to standard practice. The line cLirCcuit LC is identical with the line circuit The distributer D is mechanically a simple switch having a wiper'l76 which is operated in a forward direction only by the associated hereinafter.

In order to explain the detailed operation of the equi ment shown, it will be assumed that the su scriber at substation A desires to make a call and, accordin ly, removes his receiver from the switchhoo -When the receiver is removed at substation A, the first step coil 16 in the line circuit LC is energized over conductors 11 and 12, being connected. to conductor 11 through armature l8 and its resting contact, 1 5

and conductor 12 being grounded through the resting contact of armature 22 and said armature and the common conductor 29, which is grounded throu h the normally closed contacts controlled y armature 193, 0

This relay 82 Fig. 2. The first step coil 16, upon becoming energized over this circuit, lnagnetizes the associated core with the result that its armature, as explained in the above men-- tioned application, is operated only about half way and the first step armature'springs 19 and 20 are operated, while the second step armature springs 18, 20, 21, and 22 remain in their respective normal positions. Armature spring 19, upon being operated, disconnects test conductor 24 from the common conductor 31, and connects it to the second step or. cut-01f coil 17, and armature 20, upon being attracted, places ground upon the common start conductor 30 through the resting contact of armature 21,

and said armature. Upon the ground potential being placed upon the level start conductor- 30, the level relay 171, of which there is one for each of the ten levels of the finder banks, energizes; disconnects ground from the vertical test conductor 32 at armature 173, so as to enable the finder which is subsequently started to stop with its wipers opposite the correct level of bank contacts; and at armature 173 places ground on conductor 31, thereby grounding the test contacts of all the lines terminating in this level, with the exception of the test contacts of the line of substation A, which are not grounded owing to the fact that armature 19 of the line circuit LC is operated. As a further result of the energization of the level relay 171, it places ground on start conductor 199, which is common to the ten level relays of which relay 171 is one, thereby closing a circuit for stepping magnet 174 of the distributer D at armature 172. Upon energizing, stepping magnet 17 4 attracts its armature 175, thereby causing the pawl thereof to be advanced into engagement with the next notch in the associated ratchet wheel,.'the wiper 176, however, not being moved. As a further result of armature 175 being attracted, it places ground upon wiper 176 at contacts 198, thereby closing a circuit for the line relay of the finder F by way of start conductor 181, armature 138 and its resting contact, and armature 143 and its resting contact. Upon energizing,

line relay 106 prepares a circuit for the meter M at contacts 146; places ground on release trunk conductor 100 at contacts 145;

'and contacts 114 to battery.

contacts 109 and 114. Relay 1.02, upon energizing, connects the two windings of the shunt field test and stepping relay 103 to the release trunk conductor 160 at contacts 128 and 129; closes a locking circuit for itself at contacts 130; and at contacts 131 completes a circuit for stepping relay 103 as follows: from the grounded release trunk conductor 100, by way of contacts 128 and 129 in multiple, the two windings of relay 103, contacts 131, contacts 110, contacts 109,

Upon energizing, relay 103 places a shunt around contacts 128 and 129 at contacts 132 and 133, respectively. This however, does not have any utility during the vertical operation. As a further result of the encrgization of relay 103, it closes a circuit for vertical magnet 108 at contacts 134. This circuit extends from the grounded release trunk conductor 160, and includes armature 127 and its working contact. Vertical magnet 108, upon energizing, raises the wipers 153-158, inclusive, opposite the first level of bank contacts and, near the end of its stroke,

opens the interrupter contacts 110, thereby opening the circuit of stepping relay 103. Upon dcenergizin stepping relay 103 opens the circuit 0 vertical magnet 108 at contacts 134, whereupon vertical magnet 108 deenergizes also and again closes the circuit o1: steppin relay 103 at interrupter contacts 110. gtepping relay 103 again energizes and in this manner the said stepping relay and vertical magnet 108 operate alternately until the calling level is reached. In this case the calling level is the secpnd level, and when vertical test wiper 149 reaches the second level test contact 152, the

circuit of relay 101 is opened, whereupon said relay 101 deenergizcs and transfers the operating circuit from the vertical to the rotary magnet. This deenergization of relay 101, it will be understood, occurs at or near the end of the second vertical step and at about the same time as the second deenergization of stepping relay 103.

Upon the first energization of stepping relay 103 after the operating circuit has been transferred to rotary magnet 107, a circuit is closed at contacts 134 for said rotary magnet 107 and it rotates the switch wipers 153-158, inclusive, into engagement with the first set of bank contacts in the second level, also rotating vertical test wiper 149 away from the vertical test bank. Near the end of its stroke, magnet 107 opens at its interrupter contacts 109 the circuit of stepping relay 103 and also the circuit of relay 102. Relay 102 has now performed its last function of giving the initial rotary point has been previou y shunted by ste circuit at armature 130- isconnects the two windings of stepping re ay 103 from release trunk conductor 160 at contacts 128 and 129; and at contacts 131 opens a po nt in the circuit of steppin I relay 103, whtifih 0 vertical 01f normal contacts 111. Stepping relay 103, upon deenergizing, opens the C11- cuit of the rotary magnet 107 at armature 134, whereupon rotary magnet 107 deenergizes andlcloses its interrupter contacts 109 again.

From this point the o eration depends upon whether the contacts in which the calling line terminates are thig'lrst set of contacts or some other set. case they are the first set of contacts, the switch wipers are not rotated any further, and the finder switches through at this oint. Assuming that the contacts in whic terminates are in some set other than-the first set in the second level, testwipers 154 and 157 engage grounded contacts and,-as a result, switching rela s 105 and 104 are both short circuited am both windings of the shunt field relay 103 are energized with the result that said relay 103 aga1n attracts its armatures. This results-in locking circuits being closed for the two windings of relay 103 at contacts 132 and 133, and also in a circuit for rotary magnet 107 being closed at contacts 134. The said rotary magnet 107 again energizes with the result that the switch wipers are stepped into engagement with the second set of bank contacts, and steppin relay. 103 is again deenergized at the on of the stroke of the rotar magnet 107.

This alternate operation of stepping relay 103 and rotary magnet 107 contmues as described until the test wiper 154 engages the ungrounded test contact 27.. When this occurs, switching relay. 104, being no longer short circuited, energizes in series with the ri ht hand winding of stepping relay 103. The shunt field stepping relay 103 is not again energized when the calling set of contacts are reached notwithstanding the fact that a circuit for its left hand winding is closed through Wiper 157 and the grounded test contact 34, owing to the fact that the non-energized right hand core serves as a short circuit for the magnetic flux generated in the energized left hand core of the relay. It is true that a small'amount of flux is generated in the right hand winding of relay 1.03, but this flux is not sufficient to operate the armature of the relay owing to the fact that the current through the right hand winding of the relay 103 is limited by the high resistance winding of switchingrelay 104, through which the circuit of the right hand winding of relay 103 now extends as follows: from the grounded release trunk the calling line connects switching relay 105 at contacts 137; opens another point in the circuit of release nmMgnet 115 and closes a circuit for the 1 through contacts 146 at armameter .ture 139; and at'armature 138'opens the circuit of the slow acting line relay 106 and pre ares to extendlthe start conductor 181 tot e next finder. As a further result-of the energization of relay 104, it disconnects the test wiper 154 from the junction of switching relay 104 and the right hand winding of stepping relay 103 at armature 136 and connects it to the grounded release trunk conductor 160, thereby placing ground on the test conductor 24 through the test contact 27. When this occurs, the second step or cut off coil of the linecircuit relay becomes energized with the result that the remaining armatures of this relay are operated. Armatures 18 and 22, upon being attracted, remove. the normal battery and ground connections from the calling line, and armature 21 removes ground from start conductor 30, whereupon, the group relay 171 deenergizes. f

Relay 171, upon deenergizing,'in addition to'removing ground from the common conductor 31' and replacing ground on the group test conductor 32% armature 173,-

opens the circuit of stepping magnet'174at armature 172, whereupon stepping magnet 174 deenergizes and removes ground from Wiper 176, at the same time advancing the wiper 176 into engagement with start conductor 182 extending to the second finder. It, will now be seen that when the next call comes in, the second finder isemploycd even though the first finder F does happen to become idle in the meantime. Y

In the finder F, as a further result of the energization of switching relay 104, the line wipers 153 and 155 are connected up to conductors 159 and 161 at armatures 135 and 139', res ectively. As a result, the numerical switc connected with the finder F by the conductors 159-161, inclusive, is prepared for operation in the usual manner and, ac-

4mm I cordingly, places ground on release trunk conductor 160. This is done, it will be understood, before the slow-acting line relay 106 of the finder F has had time to deenergize.

After an interval line relay 106 deenergizes; removes ground from conductor 160 at armature 145, leaving conductor 160 grounded by the associated switch as ex-- plained above; extends the startingmircuit at armature145; opens the circuitof the call meter M at armature 146; closes a pdint in the release magnet circuit at armature 147; and at armature 148 disconnects relay 101, thereby permittingthe said relay 101 to decnergize in case it has again energized in res onse to the ground potential being replace on conductor 32 at armature 173, as above described.

After the calling subscriber has completed the desired connection in the usual mal cont-acts 113, and release magnet ,115 to battery. 'llpon ei'iergizing, release magnet 115 disconnects battery from the interrupter contacts .109 and 110 of the rotary and vertical magnets 107 and 108, respectively, at. contacts 114, so as to prevent relays 101 and 102 from becoming energized over wiper 149 during the release of the connection. As a further result of the energization of release magnet 1.15, the shaft and wipers of the finder F are restored to normal position in the usual manner, and the circuit of release magnet 115 is opened at elf-normal contacts 113 by the switch shaft when the latter reaches its normal position.

Assuming that the subscriber at substation A makes a call, and assuming further that he does so at a time when the distributer switch D is in the positionishown in the drawing and that the finder 1" is idle, the finder Fis operated in the hereinbefore described manner, with the result that the wipers 153-158, inclusive, are brought into engagement with the contacts in which the lines of substations A and A terminate. In this case, since the substation A is not the calling substation, test contact '27 is rounded and switching relay 104 of the hnder F accordingly remains short circuited. However, since the call is being made from the substation A, test contact 34 is the one which ungrounded and, accordingly, switching relay 105 is no longer shortcircuited when test wiper 157 arrives upon the said contact 34. 'That being the case, switching relay 105 energizes in series with the left-hand winding of the shunt field relay 103. Upon energizing, switching relay 105 performs at armatures 143 and 144 the same operations as were performed at armatures 138 and 13$) by switching relay 104; disconnects switching relay 104 at contacts 142; and at armature 141 disconnects test \Vhen this occurs,

The dewiper 157 from the junction of its own windin and the left-hand winding of shunt ficldLre ay 103 and connects it to the grounded release trunk conductor 160 thereby energizing the ci1t-otf windin of the two-step relay of the line circuit L As a further result of the energization of switching rela 105, it connects up the line wipers 156 an 158 at contacts 140 and 144 to the conductors .159 and 161, with the results hereinbefore described.

. It may be stated that in case the lines of substation A andA are calling simultaneously, switching relays 104 and 105 both attem t to energize when the finder has founc the calling lines. This does not cause any trouble, however,'bccause each opens the circuit of the other and they cannot both pull up. They both buzz for an instant, but due to the fact that the cannot both have absolutely the same ad ustment, one relay will-fall out of step and permit the other to pull up. The line which is not connected with this time is found by another tinder after the starting circuit has been extended to such finder.

It may somctimes occur that a subscriber will remove his receiver to make a call, but will change his mind and replace the receiver on the hook after a finder has started in search of the line and before the line has been found. Assuming that the finder F is operating in "search of a calling line, and assuming that the calling subscriber replaces his recciver before the level in which his line terminates has been reached, thereby replacing ground on the corresponding contact in the bank of test wiper 149, the finder does not sto when the callingwlevcl is reached unless, 0 course, that level is the tenth, in which case it stops on account of the fact that the switch shaft is not designed to be raised beyond the tenth level. At any rate, in case the tenth level is reached, and if at such time the test wiper 149'is in engagement with a grounded contact, a circuit is closed for release magnet 115 through off-normal contacts 1123, upon the closure of the tenth level shaft springs 118. When this occurs, release magnet 115 energizes; locks itselfto the grounded release trunk conductor 160 at contacts 116; and at contacts 114 removes battery from the interrupter contacts 105) and 110 so as to stop the voperation of vertical magnet 108 by opening the circuit of relay 101 and permitting it to deenergize. The switch does not start to rotate at this time for the reason that the circuit of stepping relay 103 is opened at contacts 114. The switch bait and wipers are now restored to normal position, and the circuit of release magnet 115 is opened in the usual manner at off-normal contacts 113, whereupon said release magnet 115 deenergizes; opens its contacts 116; and

closes its contacts 114, whereu on it is able to immediately start in searc of another calling line in case the start conductor exeleventh rotaryv step has been taken, at

which time the wipers 153-158, inclusive, will have passed one step beyond the last set of bank contacts, and the eleventh r0,- tary step springs 117 are closed, with the result that a circuit is completed for release magnet 115 from the grounded release trunk conductor 160, as a result of which the release magnet 115 energizes and restores the finder to normal.

In case all the finders are busy execept one, and assuming the idle finder is the finder F, the circuit for the slow to energize chain relay 191 is completed at all points except at off-normal contacts 112 and, accordingly, as soon as the finder F starts to operate in its vertical movement upon the next call being made, the circuit of chain relay 191 is completed at off-normal contacts 112, and relay 191 energizes; closes a circuit for the meter M at contacts 192 so as to register this condition; and at armature 193 places ground through the left-hand winding of the busy tone transformer on the common conductor 29 and removes direct ground therefrom. This is done so as to give a busy tone to a calling subscriber to inform him that no finder is at present available. The relay 191 is made slow to pull up so as to delay the placing of the busy tone on the common conductor 29 until after the finder F has had time to find the calling line. Otherwise, the calling subscriber might get a busy si nal and hang up even though the finder is searching for his line. his busy signal, of course, is meant only for those who take off their receivers at a time when all the finders are actually busy.'

Referring now to Fig. 1, the so-called discriminating tone arrangements provided for in comiection with the lines of substations A and A will be explained briefly. The buzzer 11 is constantly operating and induces a tone through the medium of the associated transformer on the common tone conductor 12. Considering the line of substation A, this tone passes through the condenser 13' and the jumper 14 to the normally open contacts 20' of the line circuit LO. These contacts are closed onl when the second step coil 17 is energize and, consequently, when the line of substation A is in actual discriminating tone is normall on the private normal conductor 14 through the contacts 20', and when a call is extended to a toll recording operator who hasm'eans for associating her headset with therelease trunk conductor, this tone is heard and indicates the class to which the line of the substation A belongs. Fora fullerdescription of the way in which the tone is listened to by the o orator, reference may be had to Fig. 8 an the accom 'anying description of the co-pendin a phcation of Ra Serial N 0. 438,985, filed Jan..21, 1921. ig. 1 of the drawings inthe. Ray case shows how any one of a plurality of characteristic tones may be placedon the private normal conductor 13 of the line switch C. The tone lead, however, is normally connected to the private normal conductor and a path'is normally (provided to battery throu h the relay 14 an stepping magnet 16. once, there is normally a load on the tone'producing machine w ich is very undesirable in a lar e exchange. It is ObVlOllS that since the discriminating tone lead is normally discon nected from the private normal conductor in the present case a considerable saving in current is effects on account of the fact that the discriminating tone current is serious objection to a system .in w ich the connected. It is common ractice to provi e some lines with tone, and to leave some lines without tone, de endin upon the respective" classes to which the lines belong. When this is done, it has been found often that the electrostatic capacity between a private normal conductor not otherwise carrying a tone and the various tone carrying private normal conductors in the same or adjacent cables is of such a value that a tone is placed on the nontone carrying private normal conductors by electrostatic induction from those carrying a tone. This induced tone is often stron enough to mislead the o erator, there y defeating the purpose of the discriminating tone arrangement. It was found that this electrostatic induction is effected principally by the rivate normal conductors of idle lines, an that no appreciable interference is caused by the tone on a private normal conductor when the "said conductor is in use, on account of the fact that it is then grounded.

It was found also that the tone placed on idle private normal conductors caused a cprresponding tone to appear on adjacent lme also, is practically eliminated when the private normal conductors aregrounded. In the present case, the electrostatic induction vconductors by electrostatic induction. This,

use,- the tone from the buzzer 11 is placed is, therefore, not present to any appreciable extent for the reason that the discriminatin tone is disconnected from a private normal conductor when the associated line is not in use and is connected to that conductor only when the associated line is in use, at which time the private normal conductor a is grounded, thereby preventing the electrostatic induction from taking place. at this time. 7

It is obvious, in any case, that with a private normal conductor grounded, due to being in use a flow of current exists in the conductor and that to a certain extent the tone will be transmitted to adjacent conductors through electromagnetic induction. However, since the amount of discriminating tone current flowing in a grounded private normal conductor is very small, there is no appreciable amount of discriminating tone current transferred in this way.

It will be seen from the foregoing that the applicant has devised a very efficient finder system and one embracing a comparatively large number of calling lines and one which has very reliab e means for extending the start circuit, which means comprises the usual chain circuit extending throu h the finders and, in addition, the distributor switch D which shifts the point of access to the starting circuit after each call. It will be seen also that there is a considerable economy effected in the consumption of discriminating tone current, and that the source of cross tones amongst the private normal conductors is removed as concerns any practical results.

What is claimed is:

1. In a telephone system, a group of lines, a group of finder switches for extending said lines when calling, a loop starting circuit extending through all said finders in series, a line relay in each finder directly controlled over said. starting circuit. con tacts on each relay for extending said circuit to the next finder, a start wire common to said lines, and a distributer switch intervenin between said start wire and said loop circuit by means of which the common start wire can be associated with said loop circuit at a plurality of different points.

2. In a telephone system, a group of lines, a group of finder switches for extending said lines when calling, a loop starting circuit extending through all said finders in series, line'and switching relays in each finder, serially related contacts on the said relays in each finder controlling the con tinuity of said starting circuit, a start wire ,common to said lines, a start wire individual to each finder switch connected to said loop circuit, and a distributor switch for associating the common start wire with any one of said individual start wires.

3. In a telephone system, a group of lines, a group of finder switches for extending said lines when calling, a loop starting circuit extending through all said finders in series, a start wire common to said lines,

a start wire individual to each finder switch connected to said. loop circuit, a distributor switch controlled over said common start wire responsive to successive calls, said distributer switch having a stepping magnet, and a circuit including contact springs directly actuated by said magnet for placing starting potentials on said individual start wires successively.

4. In a telephone system, a group of lines, a group of finder switches for extending said lines when calling, a loop starting circuit extending through all said finders in series, a start wire common to said lines. a plurality of start wires conncctcdato said loop circuit at different points, a distributor switch for associating said common start wire with any start wire of said plurality, means for advancing said distributor switch once for each call, and means for preventing such advance in case a succeeding call is made before the preceding call is located.

5. In a telephone system, a group of lines, a group of finder switches for extending said lines when calling, a loop starting circuit extending through all said finders in series, a start wire common to said lines, a plurality of other start wires connected to said loop circuit at different points, a distributer switch controlled over said common start wire responsive to a call to place a starting potential on one of said other start wires, and means for automatically advancing said distributer switch to prcsclect another of said other start wires when the calling line is found.

6. In a telephone system, a group of lines, a group of finder switches for extending said lines when calling, a start circuit extending through said finders serially over which the said finders may be started one after the other in response to successive calls, a start wire common to said lines, automatic switching means responsive to a call for changing the relation between the common start wire and the said start circuit to insure the starting of a different finder for the next call, and means preventing said switching means from responding to calls which are initiated while one or more other calls are awaiting the action of finders.

7. In a telephone system, a plurality of lines, a finder switch for extending said lines when calling, testing means in said switch for locating a calling line, said testing means comprising two test wipers and a common test relay with circuit connections for testing two of said lines simultaneously, an armature. on said test relay for controlling said circuit connections, and a magnetic circuit for said relay adapted to exert an attractive force on said armature only when both of sai'dtest. wipers encounter non-calling lines.

8. In a telephone system, a plurality of groupsof lines, a finder switch for extending said lines when calling, said switch havthe primary magnet and then with the sec ondary magnet to operate the switch as set forth.

9. In a telephone system, a plurality of subscribers lines, a trunk line comprising line conductors and a release trunk conductor, a finder switch for extending'a calling one 01' said lines to said trunk line, a

,combinationline and gout-off relay individual to the calling line for starting said finder, said relay being held energized over' said release trunk conductor after the line is found, a machine for generating signalling current capable of producing a tone indicative of the character of the calling line, and contacts on said relay closed when the same is energized for connecting the said machine with said release trunk conductor.

10. In a telephone system, a plurality of groups of lines, a finder switch for extending said lines when calling, said switch having a primary movement to'select a group of lines and having asecondary movement to select the calling line in the selected group, primary and secondary test wipers for said switch, primary and secondary operating magnets for said switch, a step- .whereby the finder is advance ping relay having impulse generating contacts, circuit arrangements local to said finder and excluding the primary test wiper for operating said stepping relay to control said primary magnet to advance the finder to the calling group, and means controlledi w over said primary test wiper for connecting said stepping relay to the secondaryv test wiper and for connecting the impulsegencrating contacts withthe secondar magnet,

stopped at the calling line.

11. In a telephone system, a plurality of roups of lines, a finder switchfor extending said lines when calling, each of said groups of lines being divided into two subgroups, said switch having a primary movement to select a group and a secondary movement to select two calling lines in the selected group, two sets oi? wipers in said switch corresponding to the two'sub-groups,

respectively, a secondary test wiper in each set of wipers, a primary test wiper, a don-i ble wound stepping relay, means for operating both windings of said stepping relay in multiple under control of said primary test wiper to drive the finder into association with the calling group, and a relay controlled over the primary test wiper to connect the two windings of the stepping relay with the secondary test wipers, respectively,

and to connect the impulse generating contacts with the secondary operating magnet, whereby the wipers are driven into engagement with the calling line.

In witness whereof, I hereunto subscribe my name this 5th day of May, A. D., 1923.

TALBOT G. MARTIN.

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